Quite a bit of emphasis is put on AFR or “Air to Fuel Ratio” on the Internet. With the emergence of ethanol blends such as E85 and the increasingly common E10 around town, any discussion AFR is not as straight forward as it would seem. AFR can only be calculated if one knows the stoich of the fuel. Most individuals do not fully understand tuning with alternative fuels and how wideband readings are affected. Some popular wideband tests could be improved from a more thorough understanding of the concepts we’re about to discuss in this two-part series.
One thing we should clear up first is the difference between your vehicles stock oxygen sensors (narrowband) and an aftermarket wideband sensor and controller. Your stock narrowband sensor is essentially a switch – it can tell you if you are higher or lower than a set point (a lambda of 1.00 – read further to understand what this is). A wideband sensor – and more importantly the controller driving it – is capable of interpreting a wide range of lambda, not just which side of a single fixed point you are on.
A discussion of widebands and air to fuel ratio must begin with an understanding of lambda (λ). So on to lambda… what is it?
Lambda is quite simply a multiplier used to determine how far off from the fuel’s stoich value you are. Lambda is the language spoken by all narrow and wideband oxygen sensors. When talking about premium unleaded gasoline, a value of around 14.64 parts of air to one part fuel is accepted as the (stoichiometric) mixture at which all fuel molecules are burned/oxidized with all oxygen molecules to form CO2 and H2O. It is generally assumed that a value of 14.64 is stoich for generic unleaded fuel which is not always the case.
Here are a few common fuel types and their approximate stoich value. Each of these stoich values are also represented as a lambda of 1.00:
- Unleaded 93 octane: 14.64
- E5 93 octane: 14.35
- E10 93 octane: 14.08
- E15 93 octane: 13.79
- VP C16: 14.77
- e85: 9.76
- e100: 9.00
Now say you want a richer mixture for a supercharged car under boost – typically I will command about 0.80 lambda. To get this into an air to fuel ratio you will take the stoichimetric value of your fuel (14.64 in this example) and multiply it by the lambda – 0.80 * 14.64 = 11.712:1. In order to use your O2 sensor and meter you must know the stoich point (14.64 used in the previous equation) if converting the O2 output to A/F and one can only use a wideband sensor for output not a narrow band sensor.
Let’s take it a bit further and check out the stock fuel table for a Shelby GT500. Understand this table is only used under certain throttle conditions but this is a good example to get you thinking…
Looking at this table, you’ll see RPM across the top and TP (throttle position) down the left. At 6000RPM and 800TP (floored) the PCM commands 0.819 lambda. What is 0.819 lambda? It is 14.64 & 0.819 = 11.99:1. The Ford PCM uses 14.64 as the stoichiometric value.
Now a caveat… just because you change this table does not mean the car will magically hit the lambda you command, in fact, there are quite a few other items that will add/subtract fueling commanded in this table. One of the many things I do while tuning your car is to make the desired fueling under all conditions match what is actually seen on the wideband, keeping in mind the various lambdas that need to be commanded during wide open throttle, cruising, idling, cold start, hot start, the list goes on.
Here’s the kicker, with the ethanol blends floating around (E5, E10, E20) the stoich of the fueling will change, changing what you see on the wideband. Scary? Yes possibly but that is why I wanted to show you a bit of what goes into properly calibrating a vehicle and what is taken into consideration, this is only the tip of the iceberg.
In my next article, the rubber meets the road. I will explain how you can datalog your air to fuel ratio/lambda, things to keep in mind while using widebands and lastly the effect of the fuel change we’ve seen here in Baton Rouge going from unleaded gasoline to an ethanol blend. For the curious I’ll also touch on my experiences with converting to e85 in non flex fuel vehicles, especially as they relate to the higher octane provided.
A special thanks goes out to Michael Rauscher at L&M Engines for proofing and offering suggestions in the development of this article. -Wes
